To control nuclear reactors, and particularly to control pressurized water nuclear reactors, absorbent units constituted by tubes containing a material which is strongly absorbent of neutrons are displaced inside the core, in the vertical direction between the fuel elements.
These control rods are introduced through the upper end of the core so that a vertical movement downwards corresponds to a reduction in the power supplied by the reactor and a vertical movement upwards corresponds to an increase in the power supplied by the reactor.
In addition, if any accident makes an emergency shutdown necessary, the control rods must be able to drop very rapidly under the action of their weight into maximum insertion position in the reactor core,
Any nuclear reactor control apparatus using the vertical displacement of absorbent units such as control rods must therefore be capable of obtaining both accurate displacement of the rods inside the core, with the possibility of stopping in a predetermined position, and instantaneous and very reliable release of these control rods for emergency shutdown.
To obtain their vertical movement, the control rods are usually connected at their upper part to a control shaft which is displaced by means of a mechanism disposed inside a sealed enclosure in communication with the interior of the reactor and containing the exchanger fluid of the reactor, for example pressurized water in the case of pressurized water nuclear reactors.
Various types of mechanism, such as pawl mechanisms and rack-and-pinion mechanisms, have already been proposed for obtaining movement of the control rods. In all cases, these mechanisms are intended to function in a high-temperature and high-pressure environment and in the presence of radiations.
French Pat. No. 1,455,472 discloses an electromagnetic motor whose movable element is constituted by the control shaft itself which bears annular projections disposed so as to form a variable reluctance motor producing a translatory movement entirely without any rotary movement.
However, it has never been possible to make good use of such an apparatus, which avoids utilizing mechanisms disposed inside an enclosure in communication with the interior of the core, because it needs too many coils to constitute the stator part of the motor, and also because it has a low efficiency as a result of magnetic saturation of the shaft due to the axial concentration of magnetic flux. In addition, the energy it consumes in keeping the movable part in position is considerable, which, among other drawbacks, leads to prohibitory overheating and necessitates providing a cooling apparatus.